It has previously been proposed to provide a ceramic insulating sleeve or bushing forming the spark plug tip exposed to combustion gases and adapted for location within the combustion chamber of an internal combustion engine of ceramic material, in which a center electrode is inserted in a longitudinal bore or opening formed in the insulating tip. The sparking tip, of metal, was inserted into the ceramic insulator before the insulator was fired, and the combination of unfired insulator ceramic, typically a press-extruded element, and the center electrode were then sintered.
The result is a combination of spark plug insulator tip and center electrode -- described, for example, in German Pat. No. 131,431 -- in which the center electrode is inserted. In manufacture, the insulating mass, while of dough-like consistency, has the insulating tip inserted therein and, thereafter, the combination of tip and ceramic dough-like material is sintered. Such a method is not specially adapted for mass production of spark plugs. It has also been proposed -- see U.S. Pat. No. 2,152,738 -- to make intermediate extruded pressed spark plug insulators by isostatic press extrusion of insulating material. Simultaneous insertion, under force, of a center electrode in a preformed press body has not, however, previously been considered.
It is an object of the present invention to facilitate and speed the method of manufacturing spark plug tips, and more particularly to render the method of manufacture more economical and to provide a resulting product which does not permit existence of an air gap between the center electrode and the insulator and thus provide for efficient heat transfer, over long lifetime, of heat from the center electrode to the ceramic material.
Briefly, in accordance with the present invention, a series of steps in the method are carried out in which, first, a movable plug element with a needle-like extension formed at the end thereof has the center electrode tip inserted therein. The movable plug element is axially movable, for example by being attached to a vertically movable press plug. The press plug is movable in an elastic, tubular press mold made, for example, of rubbery material or the like, and which can be compressed from the outside by application of hydraulic pressure, pneumatic pressure, or the like. The plug element is inserted into the rubbery press form and shifted therein until the plug closes off one end of the tubular press form. A predetermined quantity of pulverized or granular ceramic material is then filled through the upper opening of the tubular press form, the opening is then closed by an upper plug, and the entire system is pressurized from the outside so that the rubbery elastomer mold transfers the outside pressure to the insulator press element and shapes the insulator press element; the pressure from the outside is then released, the press form opened and the lower plug removed. The lower plug will then have the needle-like extension with the center electrode still inserted therein and the ceramic press element secured thereto. The ceramic press element is then removed, together with the sparking tip from the needle-like extension. The center element will remain in the end portion of the ceramic element which, later on, will form the spark tip of the spark plug.
The apparatus to carry out the method, includes the movable plug which has, at its end portion, the needle-like extension which is formed with a bore to receive the sparking tip of the spark plug, which is so dimensioned and arranged that it can hold the tip of the spark plug. The needle-like extension itself, when withdrawn from the interior of the press form will leave a longitudinal hollow space or bore within the press form to permit later inclusion of a conductor therein to conduct electricity from the connection element of the spark plug to the spark tip.
The resulting spark plug tip - ceramic insulator assembly can be made economically, and the center electrode tip is inserted in the insulator tightly and without cracks, fissures, or gaps between the metallic tip and the insulating material. Heat transfer from the insulating tip to the surrounding ceramic material is thus excellent, the heat transfer rate remaining constant over long operating time of the spark plug. Spark plugs made in accordance with the method and with the apparatus as aforesaid are particularly suitable as test spark plugs.